In recent years, in particular, tire manufacturers have made particularly strenuous efforts to develop novel ways of solving a problem dating back from the very start of the use of wheels fitted with inflated tires, namely how to allow a vehicle to continue to travel despite a substantial or total loss of pressure of one or more tires. For decades, the spare wheel was considered to be the only and universal solution. Then, more recently, the considerable advantages of possibly dispensing with the spare tire have appeared. The concept of “extended mobility” was developed. The associated techniques allow the vehicle to run with the same tire, dependent on certain limitations to be respected, after a puncture or a pressure drop. This makes it possible for example to get to a point of repair without having to stop, often in hazardous circumstances, to fit the spare wheel.
Self-sealing compositions that can achieve such an objective, which by definition are capable automatically, i.e. without any external intervention, of sealing a tire in the event of it being punctured by a foreign body, such as a nail, have been particularly difficult to develop.
To be able to be useful, a self-sealing layer must satisfy many conditions of a physical and chemical nature. It must in particular be effective over a very wide operating temperature range and over the entire lifetime of the tires. It must be capable of closing up the hole when the puncturing object remains in place and, when the latter is expelled, it must be able to fill the hole and seal the tire, especially under winter conditions.
Many solutions have been conceived, but have not been able to be developed in vehicle tires, especially owing to the lack of stability over time or the lack of effectiveness under extreme operating temperature conditions.
To help to remain effective at high temperature, document U.S. Pat. No. 4,113,799 (or FR-A-2 318 042) has proposed as self-sealing layer a composition comprising a combination of partially crosslinked butyl rubbers of high and low molecular weights, possibly in the presence of a small portion of a thermoplastic styrene elastomer. For good sealing effectiveness, said composition contains 55% to 70% by weight of a tackifier.
Document U.S. Pat. No. 4,228,839 has proposed as self-sealing layer for a pneumatic tire a rubber compound containing a first polymer material that degrades when irradiated, such as polyisobutylene, and a second polymer material that crosslinks when irradiated, preferably a butyl rubber.
Document U.S. Pat. No. 4,426,468 has also proposed a self-sealing composition for a pneumatic tire based on crosslinked butyl rubber of very high molecular weight.
A known drawback of butyl rubbers is that they suffer large hysteretic losses (high level of tan δ) over a wide temperature range, which drawback has repercussions on the self-sealing compositions themselves, giving them a large increase in hysteresis and considerably degrading the rolling resistance of tires.
The Applicants have moreover found that these butyl-rubber-based compositions may also be insufficiently effective, especially under winter temperature conditions, after the delayed expulsion or removal of a puncturing object that has remained in place for a long period of time in the pneumatic tire structure.
Document EP-B1-1 090 069 has certainly proposed self-sealing compositions containing no butyl rubber, the specific formulation of which comprises, per 100 parts by weight of a styrene-based thermoplastic elastomer, 80 to 140 parts of a liquid plasticizer, 110 to 190 parts of a tackifying resin and 2 to 20 parts of an additive.
A large quantity of tackifying resin, apart from the resulting higher industrial cost for the tires, may also degrade the rolling resistance of the tires because of the risk of excessive stiffening of the self-sealing composition.
The Applicants have now discovered, during their research, a self-sealing composition of appreciably simplified formulation, requiring neither butyl rubber nor the use of tackifying resins, which also performs better than the self-sealing compositions of the prior art.